1. Field of the Invention
The present invention relates to a composition for forming a piezoelectric, a method for producing a piezoelectric film by use of the composition, a piezoelectric element having the piezoelectric film, and an ink jet recording head.
2. Related Background Art
A piezoelectric film represented by lead zirconate titanate (PZT) is utilized as a piezoelectric element of an ink jet recording head. For forming a ferroelectric thin film for use in this application to express a sufficient electromechanical conversion capability (pressure for promoting displacement), it is necessary to promote crystallization of the film by a final heat treatment in any film formation process. Furthermore, in order to obtain a sufficient displacement which the element should possess, a thickness of about 1 μm to 25 μm is necessary.
The PZT film can usually be formed by a screen printing method, a sputtering method, a sol-gel method, a CVD method, a hydrothermal method or the like, and is usually annealed at a temperature of 700° C. or higher for obtaining crystals of a perovskite structure having piezoelectric properties. For increasing the thickness of the film, a deposition time for film formation is prolonged, or the film formation step is repeated two or more times. Among the film formation processes described above, the sol-gel method is excellent in composition controllability, and enables a thin film to be obtained easily by repeating coating and calcination. Furthermore, the film obtained by the sol-gel method is so dense that it is expected to have good piezoelectric characteristics without causing dispersion of a pressure promoting the displacement.
The sol-gel method is such that a substrate is coated with a sol containing hydrolysable compounds of constituent metals, partial hydrolysates thereof or partial polycondensates thereof as a raw material, the resultant coating film is dried and then heated in the air to form a metal oxide film, and the film is baked at a crystallization temperature of the metal oxide or higher temperature to crystallize the film to form a ferroelectric thin film. As the hydrolysable compound as a raw material, an organic compound such as a metal alkoxide, a partial hydrolysate thereof or partial polycondensate thereof is generally used. The sol-gel method enables a ferroelectric thin film to be formed most inexpensively and conveniently.
There is an organometallic decomposition process (MOD) as a process similar to the sol-gel method. The MOD process is such that a substrate is coated with a sol containing a heat decomposable organometallic compound, for example a β-diketone complex or carboxylate of a metal, the resultant coating film is heated in, for example, the air or oxygen, causing a solvent in the coating film to be evaporated and the metallic compound to be heat-decomposed to form a metal oxide film, and the film is baked at a crystallization temperature or higher temperature to crystallize the film. In this patent application, the sol-gel method, the MOD process and combinations thereof are called “sol-gel method” correctively.
Furthermore, ink jet printer heads using piezoelectric elements formed by the sol-gel method have been disclosed. For example, a methods has been disclosed in which using the sol-gel method, a sol containing a piezoelectric material is coated on a lower electrode two or more times, and the coating film is repeatedly heated to form a piezoelectric thin film of a piezoelectric element for use in the ink jet printer head (e.g., Japanese Patent Application Laid-Open No. H09-92897, Japanese Patent Application Laid-Open No. H10-139594 and Japanese Patent Application Laid-Open No. H10-290035).
Furthermore, a method of forming a piezoelectric thin film by the sol-gel method typically using a raw material (hereinafter referred to as sol) containing hydrolysable or heat decomposable organometallic compound such as alkoxides of constituent metals or hydroxides of constituent metals is well known to those skilled in the art (see Japanese Patent Application Laid-Open No. S60-236404, for example).